1. Field of the Invention
The present invention relates generally to a method of making a photonic crystal preform, having a defect, from which a photonic band-gap crystal waveguide fiber can be drawn, and particularly to a method of forming the defect of the photonic crystal preform.
2. Technical Background
Optical waveguides that guide light by the principle of total internal reflection have been in commercial use for more than two decades. Although optical waveguides of this design represent a quantum step forward in the field of telecommunication, work on alternative waveguide fiber designs continues. A particular drawback of the total internal reflection mechanism is that it acts to confine the light to a higher index portion, that is, the core, of the waveguide fiber. The higher index core is typically higher in density and so is characterized by higher attenuation due to Rayleigh scattering and by a higher non-linear coefficient. The deleterious non-linear effects can be mitigated by designing total internal reflection waveguides that have relatively high effective area. However, the complexity of the core refractive index profile usually increases for designs that provide larger effective area. This complexity usually translates into higher cost.
More recently, diffraction has been studied as a means to guide light in a material. In a light guiding protocol in which the confinement mechanism is diffraction, the material in which the light is guided, i.e., the core of the optical waveguide, can have a relatively low refractive index and thus a lower density. In fact, the use of a gas or a vacuum as a waveguide core becomes practical.
A particular structure well suited for use as a diffraction type optical waveguide is a photonic band-gap crystal. The photonic crystal itself is a regular lattice of features in which the spacing of the features is of the order of the light wavelength to be guided. The photonic crystal can be constructed of a first material having a first refractive index. Embedded in this first material, in the form of a regular lattice or array, is a second material having a second refractive index. This is the basic photonic crystal structure. Variations on this basic design can include more than two materials in the make up of the photonic band-gap crystal. The number of useful variations in the details of the lattice structure is also large.
In the basic photonic crystal structure, the xe2x80x9csecond materialxe2x80x9d can simply be pores or voids formed in the first material. That is, the voids serve as the second material in a photonic crystal. Depending upon the refractive index difference of the materials and the spatial arrangement and pitch (center to center distance between features) of the embedded features, the photonic crystal will not propagate light having a wavelength within a certain wavelength band. This is the xe2x80x9cband-gapxe2x80x9d of the photonic crystal and is the property of the photonic crystal that provides for light confinement. It is due to this property that the structure is given the name, photonic band-gap crystal.
To form an optical waveguide (or more generally, a structure that guides electromagnetic energy), a defect is formed in the photonic band-gap crystal. The defect is a discontinuity in the lattice structure and can be a change in pitch of the lattice, the replacement of a portion of the lattice by a material of different refractive index, or the removal of a portion of the photonic band-gap crystal material. The shape and size of the defect are selected to produce, that is, support, one or more modes of light propagation having respective wavelengths within the band-gap of the photonic crystal. The walls of the defect are thus made of a material, a photonic band-gap crystal, which will not propagate the mode produced by the defect.
In analogy with the total internal reflection optical waveguide, the defect acts as the waveguide core and the photonic band-gap crystal acts as the clad. However, the mechanism of the waveguide allows the core to have a very low refractive index thus realizing the benefits of low attenuation and small nonlinear coefficient.
Because of the potential benefits provided by a photonic band-gap crystal waveguide, there is a need to identify effective, low cost methods of making the photonic band-gap crystal waveguide that are tailored for use in a manufacturing operation.
One such method includes the steps of making a preform having a plurality of longitudinal passages, etching the passages to reach a desired preform geometry, and then drawing the preform into a photonic band-gap crystal waveguide fiber. Improving etching efficiency and performance of the waveguide made from the etched structure is, therefore, a primary enabling step presently under study.
One aspect of the present invention is a method of making a photonic crystal waveguide fiber preform using an assembly of a first and a second plurality of tubes. A first portion of the photonic crystal is made using a first plurality of tubes which have varying wall thickness along the circumference of the tube, where the circumference lies in a plane perpendicular to the length of the tube. It is understood that the wall thickness does not vary along the length of the tube. In assembling the tubes of this first portion of the photonic crystal, the thinner wall portions of the tubes abut one another. The second portion, i.e., the remaining portion, of the crystal is assembled using a second plurality of tubes which have a substantially constant wall thickness. The second plurality of tubes is configured to surround the first plurality of tubes. The photonic crystal so assembled is then etched until the portion of the photonic crystal where the thin walls abut is removed. Because the first plurality of tubes is assembled contiguously, etching away the wall portions where the thin walls abut has the effect of forming a defect in the photonic crystal. The first plurality of tubes is arranged so that the defect produces, i.e., supports or propagates, light modes having respective wavelengths that lie within the band gap of the photonic crystal.
In an embodiment in accord with the first aspect of the invention, the first plurality of tubes is configured so that the abutting thin walls are located relative to each other such that, if nearest neighbor abutting points are connected by a line, the line is continuous and closes upon itself. In the etching step, all of the tubes inside the continuous, closed line are removed from the photonic crystal. Advantageously the first plurality of tubes can be configured so that the line joining adjacent (nearest neighbor), thin-walled abutting points forms a symmetric geometrical shape such as a circle or a polygon.
In yet another embodiment, each member or certain selected members of the first plurality of tubes has an offset bore. That is, if one takes both the outer tube wall and the inner tube wall as each having respective symmetrical shapes, each having a center point, the two center points are spaced apart from each other. The respective inner and outer walls can be selected from a group consisting of a circle or a polygon. One advantageous choice is a hexagonal shape for the outer wall periphery and a circular shape for the inner wall periphery. The hexagonal shape for the outer wall periphery provides for a close-pack stacking of the tubes, i.e., a stacking with essentially no air gaps. The circular shape for the inner wall periphery is relatively easier to manufacture compared to a polygon shape, although this distinction is less appropriate when describing an extrusion method of making the tubes.
In yet a further embodiment in accord with the first aspect of the invention, the first plurality of tubes has a first composition different from that of the second plurality of tubes. The respective first and second compositions of the two tube types have different etch rates, the first etch rate being greater than the second etch rate. That is, the etching substance removes material of the first composition faster than material of the second composition. In this configuration, shorter etching times can be used in forming the defect in the photonic crystal.
A second aspect of the invention is a method of making a photonic band-gap crystal waveguide fiber. The first steps of this method according to the second aspect of the invention are the same as those set forth above in the first aspect of the invention wherein a photonic crystal preform is made. The additional step in making the photonic band-gap crystal waveguide fiber is to reduce the cross section of the preform until the desired diameter of the waveguide fiber is reached. This diameter reduction of the preform can be carried out using any of several techniques know in the art such as, heating and drawing the preform. As an alternative the preform can be heated and forced through a series of reducing dies. A combination of reducing dies and drawing can be used to achieve the desired diameter of the photonic band-gap crystal waveguide fiber. These techniques are described in detail in many patents and publications and will not be discussed further here.
In an embodiment of this aspect of the invention, the photonic band-gap crystal waveguide fiber has a defect which is symmetric about a center point. The symmetry of the defect in the waveguide fiber stems in large part from the symmetry of the defect in the preform achieved in the assembling and etching steps. The symmetry of the defect, which is understood to include an inner wall periphery substantially free of sharp inward projections, provides for reduced light scattering, and more uniform mode power distribution across the photonic band-gap crystal defect. Because the defect in this embodiment is a void, the defect has a lower non-linear coefficient as well. As is known in the art, the defect in the photonic band-gap crystal waveguide fiber acts as the core region, in analogy with the optical waveguide fiber based upon the principle of total internal reflection. Likewise, the photonic band-gap crystal surrounding the defect serves to confine the light mode and so is analogous to the clad of the total internal reflection type optical waveguide fiber. The modes supported by the defect have respective wavelengths xcex that lie within the band-gap of the photonic crystal.
An embodiment of the photonic band-gap crystal waveguide fiber exhibits a periphery having a regular shape. A measure of the regularity of the defect periphery can be made by comparing the relative distance between pairs of opposing points. Opposing points are points on the periphery that are located across from one another in the sense that the angle swept out in moving from one point to the other point is 180xc2x0. In this embodiment, the shortest and longest distance between pair of opposing points differ by an amount equal to about a wavelength, xcex, of the modes supported.
A third aspect of the invention is a method of making a photonic crystal waveguide fiber preform that includes the step of forming a photonic crystal having a plurality of openings separated from each other by walls. The openings extend through the photonic crystal. The walls are etched to enlarge the openings. One group of openings of the plurality of openings forms a contiguous group and has a first composition that exhibits a first etching rate. A second group of openings, essentially the remainder of the plurality, surround the first group and has a second composition that exhibits a second etch rate. The first etch rate is greater than the second etch rate so that more material is removed from the first group of openings during the etching step. The etching step thus provides a defect in the photonic band-gap crystal which can be configured to support a desired light mode (electromagnetic mode).
In an embodiment of this third aspect, respective the first and second groups of openings are formed by assembling tubes having respective first and second compositions.
In a further embodiment of this third aspect, the photonic crystal is extruded. Methods of extruding objects having a first volume of a first composition and a second volume of a second composition are known in the art. For example, a slug having a first material centrally located and a second material surrounding the first material can be forced through a die to make the photonic band-gap crystal in accord with this embodiment of the third aspect of the invention.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.